How to size your home battery backup for outages, solar integration, and energy resilience.
If you’re facing increasing grid instability or trying to establish more energy independence, you may be asking yourself, “How much battery backup do I need for my house?”
Power outages caused by extreme weather, aging infrastructure, and rolling blackouts have left homes more vulnerable, prompting homeowners to pair storage with rooftop solar or install a home battery without solar for reliable backup power and lower utility bills. In fact, U.S. home battery storage grew 64% in 2024 alone, with about half a million households now equipped to store their own electricity for use during outages or peak-rate hours.
Drawing on both industry research and real-world project data, here are some tips on how to estimate your essential daily energy needs, choose a battery size that matches your outage tolerance, understand how solar panels and storage go together, and compare options tailored for homes in both the U.S. and Canada.
In this article:
- What is a home battery backup system?
- How to determine your home’s backup battery needs
- How to calculate the right battery size for your needs
- How solar panels affect battery backup sizing
- Choosing the Home Battery That’s Right for You
- Frequently Asked Questions
What is a home battery backup system?
A home battery backup system stores electricity for later use so you can use it when the grid goes down or when utility rates are at their highest. As we mentioned above, battery backups are becoming more common as homeowners look for ways to protect themselves from outages and rising energy costs. They also help navigate changes to net metering policies, such as California’s NEM 3.0, which has lowered compensation for excess solar power sent back to the grid. Instead of exporting extra electricity, homeowners can store that energy and use it later — maximizing both savings and self-reliance.
A backup system typically includes a battery unit, an inverter to convert stored power for home use, and smart controls that manage charging and discharging. These systems charge from the grid or, if you have solar panels, from the energy your panels generate during the day. Many can be programmed to store power when rates are low — such as overnight off-peak hours or midday when solar output is high — and discharge when rates peak, helping lower your utility bills. When paired with solar, home batteries can recharge daily, reducing how much storage you need to cover multi-day outages.
A recent Stanford study found that about 63% of U.S. households could endure local or regional blackouts with a solar-plus-storage system, with storage meeting an average of roughly half their electricity needs during a blackout. At the same time, solar‑battery capacity in the U.S. surged by over 50% between 2023 and 2024, as homeowners increasingly seek resilience and savings.
How to determine your home’s backup battery needs
Sizing your home battery depends on how much energy you use, what you want to keep running during an outage, and how long you want that backup to last. Whether or not you have solar panels also changes the equation, because solar can recharge your battery during the day, reducing the total capacity needed for extended outages. Homes without solar can still charge a battery from the grid, though costs depend on your utility’s rate plan — off-peak overnight charging is usually cheapest, while daytime charging may fall into higher peak rates.
To choose the right system, you’ll want to:
- Calculate your home’s average energy use
- Decide whether to back up essential appliances or your entire home
- Plan how many hours or days you want coverage during an outage
The following sections break down each of these steps so you can estimate a battery size that fits your household’s needs. We’ll also touch on how factors like time-of-use pricing and net metering changes can influence the way you size and use your battery.
For a broader overview of how these systems work in homes, see Solar batteries for the home 101: The basics.
Calculate your home’s average energy use
Check your monthly utility bill to see how much electricity you use. Find the section called “Usage” or “Consumption.” Many bills show a daily average. But if it doesn’t, it’s pretty easy to calculate. If the bill lists 900 kilowatt-hours (kWh) for 30 days, just divide to get your daily average:
900 kWh ÷ 30 = 30 kWh per day
You can learn how to read your electric bill to locate these numbers. If you do not have a bill, use an online calculator or refer to the U.S. Energy Information Administration. Actual use changes with the size of your home, your appliances, and your climate.
Write down your peak usage, too. These are the hours when you use the most electricity. Knowing this helps if you have time-of-use rates or want to be sure the battery can handle your highest demand.
For more details, check out our guide on how many watts it takes to run a house.
Decide if you want partial or whole home battery backup
The size and cost of a battery system depends heavily on whether you choose partial or whole-home backup.
- Partial backup powers only essential items such as lights, the refrigerator, Wi‑Fi, and medical devices. This setup uses less energy and requires less capacity.
- Whole-home backup keeps everything running, including energy-intensive systems such as HVAC, electric water heaters, and laundry appliances. This option offers full comfort but may require multiple batteries, which increases costs significantly.
Defining which uses matter most to you helps avoid oversizing or undersizing your system.
Prepare for power outages and your grid-free power goals
Finally, consider how long you want your battery to cover you during an outage. Most homeowners size for 12-to-24 hours of backup, which can usually be met with one or two batteries. However, if you want 48 hours or more, you’ll need additional storage or solar panels that can recharge the battery daily. The longer you want to go, the more capacity you’ll need, so balancing outage tolerance with budget is key. These are all things you have to consider when deciding whether or not solar batteries are worth it.
How to calculate the right battery size for your needs
Once you know your daily energy use and how long you want backup power to last, you need to convert that information into actual battery capacity. Next we’ll look at how to interpret battery specs, estimate how much storage you need, and figure out how many batteries that means for your home.
Understanding battery capacity ratings
Battery manufacturers list capacity in kilowatt-hours (kWh). In simple terms, kilowatt-hours measure how much energy a battery can store. The higher the kWh rating, the longer it can power your home before needing to recharge. By matching your required total kWh to the kWh of each battery, you can see exactly how many units it would take to reach your target backup time.
However, you generally can’t use 100% of that number because batteries have limits to protect their lifespan. There are three key terms when looking at battery capacity:
- Usable capacity: The portion of the battery’s total storage you can draw on.
- Depth of discharge (DoD): The percentage of the battery’s capacity that can be safely used before recharging. A battery with a 90% DoD leaves 10% unused.
- Round-trip efficiency: The percentage of stored energy you can actually use after accounting for losses during charging and discharging.
When comparing the best solar battery options, always focus on usable capacity rather than total capacity. It provides a clearer picture of the energy you can count on during an outage.
Calculate battery capacity based on household energy usage
Begin figuring out how much storage you need by using your daily energy usage number. Then, multiply it by the number of days you want the battery to cover. After that, we recommend increasing the total slightly to account for energy lost during charging and discharging. Modern lithium-ion batteries typically offer 85–95% round-trip efficiency, meaning they lose about 5–15% of energy in the process. To simplify planning and ensure you have enough usable energy, add about 10% to your number, so that even after this loss, you’ll still meet your energy needs.
For example, if a home uses 30 kWh in a day and you want two days of coverage, multiply 30 by 2. That gives you 60 kWh. Then add 10% to be safe. That gets you to about 66 kWh. This is the amount of storage that would keep the house running for two days.
Figure out how many batteries your household needs
To figure out how many batteries you need based on the information above, divide the total capacity you need by the usable capacity of your chosen battery model. Using the 67 kWh example, a Tesla Powerwall provides about 13.5 kWh of usable capacity. Divide 67 by 13.5, and you get just under 5 — so you could need five Powerwalls to meet whole-house energy needs.
How solar panels affect battery backup sizing
Solar panels change how you size a home battery because they can recharge the battery during the day. Without solar, your battery must hold enough energy to power your home for the entire duration of the outage. With solar, the panels can refill the battery each day, so you can often meet the same backup goals with less capacity.
Key ways solar affects battery sizing include:
- Daily recharging: Panels can top off the battery during sunlight hours, reducing the amount of storage you need for multi-day outages.
- Time-of-use shifting: You can store excess solar energy when rates are low and use it during peak-rate periods to cut costs.
- Net metering changes: As some states lower net metering credits, storing and using your own solar power becomes more valuable than sending it back to the grid.
Keeping with our example from above, if your home uses 30 kWh per day and you want two days of backup without solar, you’d need around 66 kWh of storage after factoring in efficiency losses. But with solar panels that generate, say, 20 kWh per day, you could subtract that from your daily need — meaning you’d only need enough battery capacity to cover about 40 kWh of use (so, about a 44 kWh battery system). That’s a significant drop in the number of battery units you’d need, and it can lower upfront costs.
TL;DR? By factoring solar generation into your calculations, you can right-size your battery system to meet your outage goals without overspending on unnecessary capacity.
Choosing the home battery that’s right for you
As we’ve laid out, your home’s perfect battery system starts with two numbers: how much electricity you use each day and how many days you want backup power to last. Then it’s a matter of using that information to size your system, paying attention to usable capacity and efficiency losses.
If you have solar panels, you’ve got an edge, as your battery system can recharge during the day, which reduces how much storage you need. A solar-plus-storage system improves energy resilience, cuts utility bill costs, and reduces your reliance on the grid.
You don’t need to do these calculations alone, though. An expert solar installer can help you size a system that works for your needs.
Frequently asked questions
How much backup power do I need for my home?
It’s really up to you. A good rule of thumb is to get enough battery storage to cover at least 12-to-24 hours of essential use. The exact amount depends on your daily energy use and whether you plan to back up only critical circuits or your entire home. For example, if you use the U.S. average of about 30 kWh per day and want 24 hours of coverage, you’d need roughly 33 kWh of storage after efficiency losses. But with solar panels that generate 20 kWh during the day, you’d only need enough battery capacity to cover the remaining 10 kWh — significantly reducing the size and cost of your system.
How long can you run a house on battery backup?
A single battery often powers only essential devices like lights, Wi-Fi, and the refrigerator for part of a day. Adding more batteries or pairing them with solar panels can extend that coverage to multiple days. Homes that combine solar with storage may run indefinitely during outages if daily sunlight meets their energy needs.
How do I calculate the total energy consumption of my household appliances?
Check the wattage listed on each appliance and multiply by the number of hours you use it each day. Divide the result by 1,000 to convert watt-hours to kilowatt-hours. Then add up the kWh for all appliances to estimate your total daily consumption. If you simply want to know how much electricity your entire home uses, you can check your utility bill for your average daily usage.
Are there any tools or calculators available to estimate my home battery backup needs?
Yes. Many utilities and solar companies provide online calculators that use your energy data to suggest battery sizes. You can also use the daily kWh from your bill to run your own calculation following the steps in this guide.
How much does a home battery backup cost?
Home battery prices vary widely by brand, capacity, and installation complexity. Expect to pay between $10,000 and $20,000 for a single battery, including installation. The pre-installation price of backup batteries can range from $4,000 to $15,000.You may be able to offset some of these expenses with incentives or rebates. The Residential Clean Energy Credit equals 30% of the cost of new solar and battery installations, and many states and utilities offer their own programs. Your installer or local energy office can point you toward available rebates in your area.
